Gasoline, diesel fuel and heating oil are the most widely used hydrocarbons and are commonly stored underground at retail service stations, bulk service stations, municipal garages etc. Such liquid hydrocarbons are normally stored in underground tanks and there is a risk that the hydrocarbons may escape into the ground from perforations which develop on the walls of underground storage tanks.
Tank leakage problems are recognized by the petroleum marketing industry and by governmental and environmental agencies.
Furthermore, a leaking tank is generally undiscovered until a flagrant appearance of hydrocarbons is traced to it. The tracing procedure can be long and costly because of factors such as unusual soil strata, a network of backfilled trenches, large and frequent variations in the height of the water table or a dense concentration of underground tanks.
Perforations in tanks usually develop from corrosion; internal corrosion is less prevelant than external corrosion, but does occur especially in a narrow band along the tank bottom. This is because water, which condenses in the air space at the top of the tank is more dense than the hydrocarbon and sinks to form a layer at the bottom of the tank beneath the hydrocarbons. Internal corrosion is aggravated under the fill pipe because protective rust is removed by the flow of liquid against the tank bottom during filling or by the impact and movement of a gauge stick.